1. Field of the Invention
The invention relates generally to instrumentation used to measure operating conditions inside a semiconductor wafer processing chamber and, more specifically, to an apparatus for measuring pedestal temperatures by detecting infrared wavelength energy emissions from the pedestal.
2. Description of the Background Art
The materials and processes used to process a semiconductor wafer in a semiconductor wafer processing system are extremely temperature sensitive. Should these materials be exposed to excessive temperatures resulting from poor heat stabilization or transfer, yield or performance of the end product wafer may be compromised. Additionally, if process temperatures are not properly controlled, the boiling points of the metals and alloys used to create the desired circuit pathways on the wafer may be exceeded. As a result, the wafer and circuitry thereupon can be damaged. Since the pedestal is in intimate contact with the wafer and serves both as a heat sink and wafer support, measuring the temperature of the pedestal provides a good indication of the wafer temperature.
Most pedestals are cooled by conduction to a water cooled cathode base where a constant flow of water removes excess heat. Having established that the pedestal temperature is a good measure of the wafer temperature during processing, different means to measure the pedestal temperature have been used in the prior art. One method of determining pedestal temperature is to measure the temperature of the cooling water at the outlet of the cathode base. Unfortunately, this method is limited in that the temperature measured is neither an accurate nor timely measure of the pedestal temperature. The water temperature in the cathode base is measured after the heat has passed through several interfaces, conduits, and heat sinks. With unreliable, delayed data determining the proper feedback control system parameters, temperature stabilization is difficult using this technique to measure wafer temperature.
A second technique attempts to measure the pedestal temperature directly during wafer processing by placing thermocouple probes in contact with the pedestal. However, thermocouple probes placed in direct contact with the pedestal are unfortunately subjected to RF interference from a plasma used to process the wafer. As such, it is not possible to obtain an accurate temperature measurement by this approach either.
A third technique uses a temperature sensitive phosphor sensor. The sensor is mounted inside the chamber with a fiber optic cable attached to the sensor. The phosphor emits a fluorescent glow dependent upon the temperature of the phosphor. This "signal", is then transmitted by the fiber optic cable for monitoring purposes.
A fourth technique involves diffused reflectance spectroscopy. One wavelength of light reflected from the wafer surface is chosen for monitoring. A spectrometer measures the level of backscattering of the chosen wavelength (with specific energy and related temperature characteristics) to derive a wafer temperature. However, both the third and fourth techniques use extremely expensive equipment that is prone to damage in the harsh environments of a wafer processing chamber. As such, existing devices cannot provide an accurate and stable measurement of pedestal temperature that is necessary and desirable for processing semiconductor wafers.
Therefore, there exists a need in the art for an apparatus that can provide an accurate, real-time measurement of pedestal temperature with a relatively simple and rugged design to withstand a processing chamber environment.